There presently exists considerable interest in LEDs useful at about 1.3 microns because this spectral region corresponds to the dispersion minimum of the low loss silica based optical fibers which are becoming of increasing importance in optical transmission systems.
LEDs normally are preferred over lasers for use in many systems of this kind because of their lower cost, greater ruggedness and generally longer life.
Presently, for operation at 1.3 microns, a typical LED comprises a crystal comprising, in turn, a relatively thick, relatively highly doped n-type indium phosphide layer, a relatively thin relatively lightly doped n-type indium phosphide buffer layer, a still thinner lightly doped p-type indium gallium arsenide phosphide active layer and a thicker more heavily doped p-type indium phosphide confining layer. Additionally, a small dot metallic electrode makes a low resistance connection to the p-type indium phosphide back layer, and an annular metallic electrode makes low resistance connection to the n-type indium phosphide front layer. The limited area dot electrode serves to confine the current flow through the crystal and the consequent region of light emission.
Unlike a laser where the stimulated emission is highly directional parallel to the layers, in an LED the spontaneous emission is omnidirectional. It has generally been believed important for high efficiency to provide for the reflection of light emitted initially in the direction of the dot-contact surface back towards the annular-contact front surface for capture there by an optical fiber whose input end is butted to the central portion of this front surface. Typically, the diameter of the dot is made slightly smaller than the core diameter of the fiber so that as much as possible of the light generated is captured by the input end of the fiber.
Similarly, to maximize the capture of generated light, it has been the practice to avoid light absorptive material in the crystal in the path of useful light, including light back-scattered from the back surface.
In LEDs, especially of the quaternary type, it is desirable to minimize series resistance to minimize heating effects which are deleterious to long useful operation. In particular, it is important to keep low the contact resistance at the electrode connections, particularly at the dot contact which has a smaller area and so tends to be the main source of the contact resistance.